Personal favorite, regardless of actual quality? The answer used to be Phenomena. But... now it's:

And there are so many scenes that mean more to me in this film than the ones from other Argento films. But, even then, it's the ending (especially the moment where the men try to get her to tell them something about the killer, since she'd spent enough time alone with him to get to where she was the only person who knew his motivation, something illuminating and she only engages them when they ask her about when she knew they were coming) that cements this as Argento's least deeply understood film. It's such a strangely hopeless film. I guess Cristina Marsillach gave a very nervy, batty performance and went against Argento's direction nearly every step of the way but she still communicates a wealth of disconnect. It's easily Argento's smartest film about human relationships. Not to mention all the wild, technically off-the-charts stuff he does with the camera. Betty is a much more fascinating character than almost any 80's horror "heroine."

Argento is just in another league when it comes to horror. His best work is so damn sublime. I can't believe the films are as good when I watch them either.